Object Oriented Programming. Week 9 Part 1 File I/O

Transcription

1 Object Oriented Programming Part 1 File I/O

2 Lecture Overview of Files Using Tests to learn Java Writing Text Files Reading Text Files 2

3 Overview of Files 3

4 Overview of Files How they are accessed: sequential: data items must be accessed in the order in which they are stored (ie. start at the beginning and pass through all the items) direct (or random): items are accessed by specifying their location How information is represented: text files: data is stored in character form. binary files: data is stored in internal binary form (faster and more compact than text files). 4

5 Types of Files Summer.txt Rough winds do shake the darling buds of May\n And Summer s lease hath all too short a date\n Sometime too hot the eye of heaven shines,\n And oft is his gold complexion dimmed,\n And every fair from fair sometime declines... Numbers.dat Œpôw10Žé l % ú 9câÜ(3xLenfˆx¹ª(Ͻ»¼øß: µ œœýçmù¾à:ˆqfõ Ñ> èœ=l... 5

6 Text Files Text file human-readable with simple tools (Notepad, Ready,...) Each line terminated by end-of-line marker ( \n ) Example:.java files Easy to read and write text files Advantage of "streams" approach to I/O Use same classes and methods as System.in and System.out 6

7 Text Files: The File Class Need File object for each file program uses File infile = new File( Summer.txt"); Purpose Contains information about the file A go-between for the file Not the same as the file itself 7

8 Text Files: The File Class Methods in the File class exists(): Tells if the file is there canread(): Tells if program can read the file canwrite(): Tells if program can write to the file delete(): Deletes the file isdirectory(): Tells if file is really a directory name isfile(): Tells if the file is a file (not a directory) length(): Tells the length of the file, in bytes 8

9 Using Tests to Explore Java 9

10 Learning from Tests To learn to use a new element of java, you need to use it But, you risk forgetting how it works if you do not use it frequently By putting your exploratory code in your test directory, you can develop examples of how a new feature works. 10

11 Tests as Specifications Running unit tests show that your code works But the code in the tests indicate what your code does The assert statements tell the reader that value to expect when making a calls They provide examples of working code Unit tests are specifications of the class, as well as tests of the class What's more, they fail if the class does not meet the specification 11

12 Tests as specifications of Java You can add specifications of the crucial features of Java to your tests. Create unit tests that provide examples of Java classes you use Writing such a specification Gives you examples if as you build your code Gives you warning if Java specifications change Indicate to others what elements you are using 12

13 Text Files: The File Class Lets build tests of the following methods exists(): Tells if the file is there canread(): Tells if program can read the file canwrite(): Tells if program can write to the file delete(): Deletes the file isdirectory(): Tells if file is really a directory name isfile(): Tells if the file is a file (not a directory) length(): Tells the length of the file, in bytes 13

14 First we need to create a file 14

15 Import java.io.file java.io.file 15

16 Test that a file was created Create a file and check that it was created 16

17 Test exists() The file does not yet exist 17

18 Refactor file creation to Before method File f changed to field in class Created method Removed file creation Removed file creation Still works 18

19 Use createnewfile() to tests exists() f.createnewfile creates empty file Throws IOException 19

20 Problem: Fails Second Time First Time Second Time The testexists test fails Assertion that the file does not exists fails 20

21 Solution: Clean up file after test Import After tag method Delete f after every test It is safe to delete f after every test because we create it before every test. 21

22 Now it works every time First Time Second Time Third Time 22

23 Idempotence Any time you take an action that changes the state of the computer in a test, make sure to undo that action before completing the test Idempotence means that multiple applications of a function or method should yield the same results Tests must be idempotent Methods are guaranteed to be idempotent if they change nothing while running 23

24 Test File: tests exists() and delete() 24

25 Writing to Text File 25

26 Text Files: Writing to a File Before writing, make sure either... File doesn't exist!outfile.exists() Or file exists, and is writeable outfile.exists() && outfile.canwrite() Combine conditions!outfile.exists() outfile.canwrite() 26

27 Text Files: Writing to a File Attach file to a stream PrintStream object: knows how to write stream to a file 27

28 Creating a PrintStream from a file PrintStream File 28

29 Text Files: Writing to a File Once finished writing to the file, close it pwriter.close(); 29

30 Test Printing Create a PrintStream from a File Constructor throws error if directory Or cannot be opened Close file after printing Here we only test that we can open a file for printing and print to the file. We do not test that the character are actually in the file. We will test that when we test read. 30

31 Reading From Test Files 31

32 Text Files: Reading from a File Before reading, make sure File exists and is readable infile.exists() && infile.canread() Attach file to a stream FileReader object: knows how to read stream from a file Wrap FileReader object in BufferedReader object BufferedReader works on files just like System.in read(): read a single character, or -1 if EOF readline(): read a line, or null if EOF 32

33 BufferedReader FileReader File 33

34 Plan to Test File Reading Copy the previous test to write test in file Create a BufferedReader from the same file Call readline() on the BufferedReader Check that we get test back 34

35 Testing Reading Create a PrintStream from a File Print test Create BufferedReader form same File Read a line from the file Check that string read is string written 35

36 Note: error message from catch The error messages in the catch block are adapted from the Java documentation The provide information on why the error might have been thrown We can use that information as part of our documentation of the feature we are using It indicates in the code, why the error might have been thrown. 36

37 Refactoring: Extracting Print from Tests We now have identical code in two tests, and indication that we should refactor Eclipse has a function Refactor > Extract Method that will turn a block of code into a method. In addition we will add a parameter of the string we want to write. 37

38 Pull out PrintStream ps = null First we need to make the PrintStream we are using a field because we use it in two places New field 38

39 Extract Method Highlight code to extract Right click Refactor Extract Method 39

40 Give the method a name Name method 40

41 Result 41

42 Fix testread() to use printtofile() 42

43 Tests still run 43

44 Refactoring Improvements We have made the code clearer We have reduce the amount of code The name of the method indicates what it does The only line of code you can be certain has no bug is the one that isn't there. We can change the behavior of printing in all of the tests at the same time 44

45 Giving printtofile a parameter Let's change the method so we can pass in a string to test. The advantage is we can pass a string in to be written, then check that we get the same string back The payoff is in testread() 45

46 Changing testwrite() Called with String parameter String parameter added to signature String parameter printed 46

47 Changing testread() Variables to read and write Print write string Read read string Assert written and read are same 47

48 Advantages of Parameter refactoring testread() is clearer String written and String read are same We may be able to use the method in other tests 48

49 Text Files: Reading from a File Can do same things we did with System.in Read numbers (NumberFormat) Read multiple tokens (StringTokenizer) 49

50 Java I/O Summary 50

51 Java I/O Summary Reading from Keyboard BufferedReader( InputStreamReader( System.in )) Writing to Screen System.out Reading from File BufferedReader( FileReader( File )) Writing to File PrintStream( File ) 51